Unraveling the role of land use and microbial activity in shaping dissolved organic matter characteristics in stream ecosystems

Surface water samples were collected from 43 streams distributed throughout watersheds of mixed land use in southern Ontario, Canada. Absorbance and fluorescence spectroscopy with parallel factor analysis (PARAFAC) was used to characterize dissolved organic matter (DOM). DOM characteristics were related to environmental variables, microbial activity indicators (bacterial production and extracellular leucine aminopeptidase activity), and riparian land use to understand better how these factors influence DOM in streams. PARAFAC produced a six‐component model (C1 to C6). Temperature correlated with each PARAFAC component, suggesting that water source, drainage area, and light penetration broadly affected DOM characteristics. C1 and C2 represented terrestrial, humic‐like DOM fluorophore groups and comprised 41–65% of stream DOM fluorescence. C5, a tryptophan‐like component, related negatively to a humification index but positively to leucine‐aminopeptidase activity and recently produced DOM, suggesting that C5 consisted of autochthonous, microbially produced DOM. C3, C4, and C6 showed signs of quinone‐like, humic‐like, and microbial transformable fluorophores. The distribution of these potentially redox‐active PARAFAC components indicated that DOM was in a more reduced state in streams with higher bacterial production and agricultural land use than in streams with increased wetlands area, which had greater relative abundance of the oxidized quinone‐like component. Anthropogenic land use and microbial activity altered the quantity and quality of DOM exported from human‐affected streams from that observed in forest‐ and wetland‐dominated streams. DOM in agriculturally affected streams was likely more labile and accessible to the microbial community than DOM in wetland streams, which supported low rates of microbial activity.